Some minor modifications to classes Vessel, VMS and Driver

1)Vessel.java:
- Removed variable iteration;
- Modified definitions of methods getPosition() and setNextPosition().

2)VMS.java:
- Changed arrays highRisk[] and lowRisk[] to a single array risk[];
- Modified setting of new boat positions in updateBoats();
- Modified algorithm to set risk levels in updateDistance().

3)Driver.java:
- Modified third parameter passed in timer.schedule() method call;
- Modified UpdateTask.run() method definition to interrupt after total time elapsed.